Electro-pneumatic systems allow the implementation of electrical control systems by converting electrical signals to a pressure value at the location of the pneumatic mechanism being operated. In conventional electro-pneumatic pressure converters, electrical input signals are used to regulate valves to attain and maintain a desired pressure value for the pneumatic actuator or device connected to the valve.
Electro-pneumatic transducers are typically utilized with devices such as damper actuators, control valves, positioners, step controllers and switching devices. Typically, the electro-pneumatic pressure converter can be used as an interface device to provide a means to control activation of simple pneumatically actuated machinery or for implementing complex HVAC control strategies, directing fluid control systems, and controlling robotic systems and the like with pneumatic devices. Generally, these pneumatic devices require the use of costly transducers which consume large amounts of energy and require significant maintenance.
Electro-pneumatic transducers have been based on two conventional configurations. One configuration utilizes a modulating two-way valve with its input connected to a pressure source through a fixed small opening or restriction and an exhaust output connected to the atmosphere surrounding the device. Varying the effective restriction through the valve results in a variation of the pressure drop across the valve, which typically serves as a device output signal. The disadvantages of such a configuration include an inherent trade-off between output capacity and air consumption, a significant increase in device air consumption as output approaches atmospheric pressure and the inability to effectively control devices having output near to and including atmospheric pressure due to the finite restriction through a full open valve system. A pneumatic relay or amplifier stage is often included in these configurations in order to minimize the effect of these and other difficulties. However, the additional features do not sufficiently diminish the problem, and often actually substantially increases the complexity, size and overall cost of the device.
Yet another known configuration replaces the fixed restriction with an additional modulating two-way valve. These two-way valves are generally referred to as the supply and exhaust valves. In these configurations, changes in output pressure are achieved by simultaneously varying the effective restriction through both valves. Thus, increases in output pressure are generated by simultaneously decreasing the effective restriction through the supply valve on the input pressure source while increasing the restriction through the exhaust valve to atmosphere. Conversely, decreases in output pressure are achieved by increasing the effective restriction through the supply valve simultaneously while decreasing the effective restriction through the exhaust valve.
Conventional miniature valves have utilized direct solenoid actuation. Typically, a three-way solenoid valve provides two distinct states of operation with respect to a common port where either of two input ports may be fluidly connected to the common port. One state of operation requires that the solenoid be de-energized in order to provide a fluid path between the normally open input port and the common port. The second state of operation requires that the solenoid be energized to provide a fluid path between the normally closed input port and the common port and also to eliminate the fluid path between the normally open input port and the common ports. The recognized disadvantages of direct solenoid actuation include high power consumption, high costs, low switching speed, limited ambient temperature range for continuous operation (heat buildup), vibration sensitivity, and limited cycle rating. Another less commonly cited disadvantage of direct solenoid actuation is that the high power levels present in these valves tends to heat the controlled media being directed through the valve, which can be highly undesirable for applications such as in the biomedical field where the fluids being metered may be highly temperature sensitive.
Piezoelectrically actuated valves are generally recognized as offering a lower cost solution to the disadvantages of electro-magnetic devices that have been outlined above. For instance, in U.S. Pat. No. 4,617,952 to Fujiwara, which issued Oct. 21, 1986, a typical piezo element mounting configuration that provides a three-way valve action is illustrated. In Fujiwara a three-way valve utilizing two piezo elements or a single piezo element is described. The dual element approach offers the advantage of a quiescent control state where both piezo elements are sealed against the respective valve nozzles, which effectively disconnects the output chamber from both the supply and exhaust passages and thereby minimizes air consumption.
The presently available single piezo element configurations, such as that disclosed in Fujiwara, have at least one serious drawback in that they do not allow a quiescent condition in which the supply and exhaust passages are sealed with respect to the output chamber. This drawback also results in the further disadvantage of significant air consumption or in restricted output capacity, especially when such valves are used in control schemes requiring continuous modulation of the pneumatic output signal.
Thus, there is a need for a three-way piezoelectric valve which is simple in construction and reasonably affordable to manufacture and which reduces costs by limiting air consumption. Additionally, there is a need for a three-way piezoelectric valve which uses a single piezo element and which allows for a quiescent condition in which the supply and exhaust passages are sealed with respect to the output chamber once a desired output pressure has been attained.
Accordingly, it is an object of the present invention to provide a single element, three-way piezoelectric valve having three distinct states of operation: increase pressure state, decrease pressure state and a quiescent pressure state.
It is a further object of the present invention to provide a three-way piezoelectric valve which utilizes a single piezo element which actuates two normally closed valves with respect to an output chamber.
It is yet another object of the present invention to provide a three-way piezoelectric valve which is low cost, small in size and which provides high switching speed with limited self-heating while being energy efficient and vibration insensitive.